Fullerenes are spheroidal, closed-cage molecules consisting essentially of sp2-hybridized carbons typically arranged in hexagons and pentagons. Fullerenes, such as C60, also known as Buckminsterfullerene, more commonly, “buckyballs,” and C70, have been produced from vaporized carbon at high temperature. Presence of a transition metal catalyst with the high temperature vaporized carbon results in the formation of single-wall tubular structures that may be sealed at one or both ends. These carbon cylindrical structures, known as single-wall carbon nanotubes or, commonly, “buckytubes” have extraordinary properties, including both electrical and thermal conductivity and high strength.
Nested single-wall carbon cylinders, known as multi-wall carbon nanotubes, possess properties similar to the single-wall carbon nanotubes; however, single-wall carbon nanotubes have fewer defects, rendering them stronger, more conductive, and typically more useful than multi-wall carbon nanotubes. Single-wall carbon nanotubes are believed to be much more free of defects than are multi-wall carbon nanotubes because multi-wall carbon nanotubes can survive occasional defects by forming bridges between the unsaturated carbon of the neighboring cylinders, whereas single-wall carbon nanotubes have no neighboring walls for defect compensation. Single-wall carbon nanotubes are individual molecules of carbon, and their chemistry and their interactions with materials are fundamentally different from that of multi-wall carbon nanotubes.
In defining the size and conformation of single-wall carbon nanotubes, the system of nomenclature described by Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19, will be used. Single-wall tubular fullerenes are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When n=m, the resultant tube is said to be of the “arm-chair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. When m=0, the resultant tube is said to be of the “zig zag” or (n,0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig zag pattern. Where n≠m and m≠0, the resulting tube has chirality. The electronic properties are dependent on the conformation, for example, arm-chair tubes are metallic and have extremely high electrical conductivity. Other tube types are metallic, semi-metals or semi-conductors, depending on their conformation. Regardless of tube type, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
Several methods of synthesizing fullerenes have developed from the condensation of vaporized carbon at high temperature. Fullerenes, such as C60 and C70, may be prepared by carbon arc methods using vaporized carbon at high temperature. Carbon nanotubes have also been produced as one of the deposits on the cathode in carbon arc processes.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus by simultaneously evaporating carbon and a small amount of Group VIIIB transition metal from the anode of the arc discharge apparatus. Arc discharge methods produce only small amounts of carbon nanotubes, intermingled with other non-nanotube carbon forms, with the population of these carbon nanotubes exhibiting significant variations in size and structure.
Another method of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms, such as nickel, cobalt, or mixtures thereof, to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be in clusters, termed “ropes,” of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular arrangement. Nanotubes produced by this method vary in structure, although certain structures tend to predominate in some circumstances. Although the laser vaporization process produces an improved yield of single-wall carbon nanotubes, the product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials. In addition, the laser vaporization of carbon is a high-energy process, requiring substantial power input for vaporization of graphite.
Another way to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules dissociate on the metal particle surface and the resulting carbon atoms combine to form nanotubes. The method typically produces imperfect multi-walled carbon nanotubes, but under the certain reaction conditions, can produce excellent single-wall carbon nanotubes. One example of this method involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 catalyzed by transition metal catalyst particles comprising Mo, Fe, Ni, Co, or mixtures thereof residing on a support, such as alumina. Although the method can use inexpensive feedstocks and moderate temperatures, the yield of single-wall carbon nanotubes can be low, with large amounts of other forms of carbon, such as amorphous carbon and multi-wall carbon nanotubes present in the product. The method often results in tangled carbon nanotubes and also requires the removal of the support material for many applications.
All-gas phase processes can be used to form single-wall carbon nanotubes. In one example of an all gas-phase process, single-wall carbon nanotubes are synthesized using benzene as the carbon-containing feedstock and ferrocene as the transition metal catalyst precursor. By controlling the partial pressures of benzene and ferrocene and by adding thiophene as a catalyst promoter, single-wall carbon nanotubes can be produced. However, this method suffers from simultaneous production of multi-wall carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature conditions necessary to produce high quality single-wall carbon nanotubes.
Another method for producing single-wall carbon nanotubes involves an all-gas phase method using high pressure CO as the carbon feedstock and a gaseous transition metal catalyst precursor. (“Gas Phase Nucleation and Growth of Single-Wall Carbon Nanotubes from High Pressure Carbon Monoxide,” International Pat. Publ. WO 00/26138, published May 11, 2000, incorporated by reference herein in its entirety). This method permits continuous nanotube production, and it has the potential for scale-up to produce commercial quantities of single-wall carbon nanotubes. This method is also effective in making single-wall carbon nanotubes without simultaneously making multi-wall nanotubes. Furthermore, the method produces single-wall carbon nanotubes in high purity, such that less than about 10 wt % of the carbon in the solid product is attributable to other carbon-containing species, which includes both graphitic and amorphous carbon. Although the carbon nanotubes from this product are of high quality and purity, there is still a need for a method for producing single-wall carbon nanotubes at higher catalyst productivity and feedstock yields in order to improve the process economics and produce high quality single-wall carbon nanotubes at lower cost.
The search for methods to produce single-wall carbon nanotubes at high yield and high catalyst productivity has been an on-going need in order to make nanotubes economically viable in various applications. Besides improving economics, higher process and catalyst yields provide routes to increased availability of larger amounts of single-wall carbon nanotubes. In conventional chemical processes, higher product yields can often be achieved with higher temperatures, pressures, catalyst and feed concentrations. Contrary to convention, these techniques have not been effective in making single-wall carbon nanotubes in the gas phase using CO as the carbon-containing feedstock. Although somewhat higher yields are observed at higher pressures, the higher yields can often be attributed to higher associated catalyst concentrations. In the gas phase, using CO as the carbon-containing feedstock, single-wall carbon nanotube yield decreases at temperatures above 1050° C., possibly due to metal cluster evaporation and chemical attack of the metal catalyst clusters by CO. (See Bronikowski, et. al., J Vac. Sci. Technol. A 19:1800 (2001)).
Although transition metal catalyst particles formed from the dissociation of the transition metal precursors can result in high quality single-wall carbon nanotubes, higher yields are confounded by inherent limitations in metal clustering. Obstacles to transition metal clustering include, among other factors, low metal-metal binding energies, metal evaporation and chemical attack by CO to form metal carbonyls. In the case of iron, a preferred transition metal catalyst, the binding energy for a Fe—Fe dimer is relatively low and on the order of 1 eV, and the electronic structure of the free iron atoms is not highly conducive for the formation of dimers from the association of free atoms. Low binding energies and electronic structure makes initial nucleation more difficult and metal evaporation more problematic.
Metal clustering dynamics can be improved to some extent by nucleating the clusters using other transition metals having higher metal-metal binding energies. For example, nickel, which has a metal-metal binding energy on the order of 2 eV, or approximately twice that of iron, can be introduced into the reactor through the dissociation of another catalyst precursor, such as nickel tetracarbonyl. The presence of nickel is expected to facilitate nucleation and growth of clusters comprising both iron and nickel. Although nucleation may be improved, use of different metal species can affect the characteristics of the single-wall carbon nanotubes formed as well as the nanotube yield. Generally, the addition of other transition metals to assist cluster nucleation and growth can provide some improvement in single-wall carbon nanotube production. However, generally, the use of mixtures of iron and other transition metals does not result in substantial yield enhancements.
Not meaning to be held by theory, the formation of stable transition metal catalyst clusters of the appropriate size can be a rate-limiting step in the production of single-wall carbon nanotubes. Furthermore, in a high pressure reactor operating with very short residence times, on the order of a few hundred milliseconds in a “once-through” mode, the need for fast, stable metal clustering becomes even more critical to high yield production of single-wall carbon nanotubes. Longer residence times may enhance the carbon nanotube yield, but longer residence times also allow the metal clusters to grow large through Oswald ripening. If the cluster diameter exceeds about 3 nanometers, it becomes overcoated with amorphous and graphitic carbon, as the growth of these forms of carbon are energetically favored on large metal clusters compared to the formation of single-wall carbon nanotubes. Metal catalyst particles that are too large to grow single-wall carbon nanotubes therefore contribute to a poor ratio of carbon nanotube product to catalyst.
Accordingly, there remains a need for a method for producing single-wall carbon nanotubes at high yield and high catalyst productivity.